Optical fibers are typically coated with two or more radiation curable coatings. These coatings are typically applied to the optical fiber in liquid form, and then exposed to radiation to effect curing. The type of radiation that may be used to cure the coatings should be that which is capable of initiating the polymerization of one or more radiation curable components of such coatings. Radiation suitable for curing such coatings is well known, and includes ultraviolet light (hereinafter “UV”) and electron beam (“EB”). The preferred type of radiation for curing coatings used in the preparation of coated optical fiber is UV.
The coating which directly contacts the optical fiber is called the Primary Coating, and the coating that covers the Primary Coating is called the Secondary Coating. It is known in the art of radiation curable coatings for optical fibers that Primary Coatings are advantageously sorter than Secondary Coatings. One advantage flowing from this arrangement is enhanced resistance to microbends.
Microbends are sharp but microscopic curvatures in an optical fiber involving local axial displacements of a few micrometers and spatial wavelengths of a few millimeters. Microbends can be induced by thermal stresses and/or mechanical lateral forces. When present, microbends attenuate the signal transmission capability of the coated optical fiber. Attenuation is the undesirable reduction of signal carried by the optical fiber. The relatively soft Primary Coating provides resistance to microbending of the optical fiber, thereby minimizing signal attenuation. The relatively harder Secondary Coating provides resistance to handling forces such as those encountered when the coated fiber is ribboned and/or cabled.
The article, “UV-CURED POLYURETHANE-ACRYLIC COMPOSITIONS AS HARD EXTERNAL LAYERS OF TWO-LAYER PROTECTIVE COATINGS FOR OPTICAL FIBRES”, authored by W. Podkoscielny and B. Tarasiuk, Polim.Tworz.Wielk, Vol. 41, Nos. 7/8, p. 448-55, 1996, NDN-131-0123-9398-2, describes studies of the optimization of synthesis of UV-cured urethane-acrylic oligomers and their use as hard protective coatings for optical fibers. Polish-made oligoetherols, diethylene glycol, toluene diisocyanate (Izocyn T-80) and isophorone diisocyanate in addition to hydroxyethyl and hydroxypropyl methacrylates were used for the synthesis. Active diluents (butyl acrylate, 2-ethylhexyl acrylate and 1,4-butanediol acrylate or mixtures of these) and 2,2-dimethoxy-2-phenylacetophenone as a photoinitiator were added to these urethane-acrylic oligomers which had polymerization-active double bonds. The compositions were UV-irradiated in an oxygen-free atmosphere. IR spectra of the compositions were recorded, and some physical and chemical and mechanical properties (density, molecular weight, viscosity as a function of temperature, refractive index, gel content, glass transition temperature, Shore hardness, Young's modulus, tensile strength, elongation at break, heat resistance and water vapor diffusion coefficient) were determined before and after curing.
The article, “PROPERTIES OF ULTRAVIOLET CURABLE POLYURETHANE-ACRYLATES”, authored by M. Koshiba; K. K. S. Hwang; S. K. Foley; D. J. Yarusso; and S. L. Cooper; published in J. Mat. Sci., 17, No. 5, May 1982, p. 1447-58; NDN-131-0063-1179-2; described a study that was made of the relationship between the chemical structure and physical properties of UV cured polyurethane-acrylates based on isophorone diisocyanate and TDI. The two systems were prepared with varying soft segment molecular weight and cross linking agent content. Dynamic mechanical test results showed that one- or two-phase materials might be obtained, depending on soft segment molecular weight. As the latter increased, the polyol Tg shifted to lower temperatures. Increasing using either N-vinyl pyrrolidone (NVP) or polyethylene glycol diacrylate (PECDA) caused an increase in Young's modulus and ultimate tensile strength. NVP cross linking increased toughness in the two-phase materials and shifted the high temperature Tg peak to higher temperatures, but PEGDA did not have these effects. Tensile properties of the two systems were generally similar.
Typically in the manufacture of radiation curable coatings for use on Optical Fiber, isocyanates are used to make urethane oligomers. In many references, including U.S. Pat. No. 7,135,229, “RADIATION-CURABLE COATING COMPOSITION”, Issued Nov. 14, 2006, assigned to DSM IP Assets B.V., column 7, lines 10-32 the following teaching is provided to guide the person of ordinary skill in the art how to synthesize urethane oligomer: Polyisocyanates suitable for use in making compositions of the present invention can be aliphatic, cycloaliphatic or aromatic and include diisocyanates, such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 1,3-xlylene diisocyanate, 1,4-xylylene diisocyanate, 1,5-naphthalene diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, 3,3′-dimethyl-4,4′-diphenylmethane diisocyanate, 4,4′-diphenylmethane diisocyanate, 3,3′-dimethylphenylene diisocyanate, 4,4′-biphenylene diisocyanate, 1,6-hexane diisocyanate, isophorone diisocyanate, methylenebis(4-cyclohexyl)isocyanate, 2,2,4-trimethylhexamethylene diisocyanate, bis(2-isocyanate-ethyl)fumarate, 6-isopropyl-1,3-phenyl diisocyanate, 4-diphenylpropane diisocyanate, lysine diisocyanate, hydrogenated diphenylmethane diisocyanate, hydrogenated xylylene diisocyanate, tetramethylxylylene diisocyanate and 2,5(or 6)-bis(isocyanatomethyl)-bicyclo[2.2.1]heptane. Among these diisocyanates, 2,4-toluene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, and methylenebis(4-cyclohexylisocyanate) are particularly preferred. These diisocyanate compounds are used either individually or in combination of two or more.
Microbends are sharp but microscopic curvatures in an optical fiber involving local axial displacements of a few micrometers and spatial wavelengths of a few millimeters. Microbends can be induced by thermal stresses and/or mechanical lateral forces. When present, microbends attenuate the signal transmission capability of the coated optical fiber. Attenuation is the undesirable reduction of signal carried by the optical fiber.
The relatively soft inner Primary Coating provides resistance to microbending which results in attenuation of the signal transmission capability of the coated optical fiber and is therefore undesirable. Microbends are sharp but microscopic curvatures in the optical fiber involving local axial displacements of a few micrometers and spatial wavelengths of a few millimeters. Microbends can be induced by thermal stresses and/or mechanical lateral forces. Coatings can provide lateral force protection that protects the optical fiber from microbending, but as coating diameter decreases the amount of protection provided decreases. The relationship between coatings and protection from lateral stress that leads to microbending is discussed, for example, in D. Gloge, “Optical-fiber packaging and its influence on fiber straightness and loss”, Bell System Technical Journal, Vol. 54, 2, 245 (1975); W. B. Gardner, “Microbending Loss in Optical Fibers”, Bell System Technical Journal, Vol. 54, No. 2, p. 457 (1975); T. Yabuta, “Structural Analysis of Jacketed Optical Fibers Under Lateral Pressure”, J. Lightwave Tech., Vol. LT-1, No. 4, p. 529 (1983); L. L. Blyler, “Polymer Coatings for Optical Fibers”, Chemtech, p. 682 (1987); J. Baldauf, “Relationship of Mechanical Characteristics of Dual Coated Single Mode Optical Fibers and Microbending Loss”, IEICE Trans. Commun., Vol. E76-B, No. 4, 352 (1993); and K. Kobayashi, “Study of Microbending Loss in Thin Coated Fibers and Fiber Ribbons”, IWCS, 386 (1993). The harder outer Primary Coating, that is, the Secondary Coating, provides resistance to handling forces such as those encountered when the coated fiber is ribboned and/or cabled.
Optical fiber Secondary Coating compositions generally comprise, before cure, a mixture of ethylenically-unsaturated compounds, often consisting of one or more oligomers dissolved or dispersed in liquid ethylenically-unsaturated diluents and photoinitiators. The coating composition is typically applied to the optical fiber in liquid form and then exposed to actinic radiation to effect cure.
In many of these compositions, use is made of a urethane oligomer having reactive termini and a polymer backbone. Further, the compositions generally comprise reactive diluents, photoinitiators to render the compositions UV-curable, and other suitable additives.
Published PCT Patent Application WO 2005/026228 A1, published Sep. 17, 2004, “Curable Liquid Resin Composition”, with named inventors Sugimoto, Kamo, Shigemoto, Komiya and Steeman describes and claims a curable liquid resin composition comprising: (A) a urethane (meth)acrylate having a structure originating from a polyol and a number average molecular weight of 800 g/mol or more, but less than 6000 g/mol, and (B) a urethane (meth)acrylate having a structure originating from a polyol and a number average molecular weight of 6000 g/mol or more, but less than 20,000 g/mol, wherein the total amount of the component (A) and component (B) is 20-95 wt % of the curable liquid resin composition and the content of the component (B) is 0.1-30 wt % of the total of the component (A) and component (B).
Many materials have been suggested for use as the polymer backbone for the urethane oligomer. For example, polyols such as hydrocarbon polyols, polyether polyols, polycarbonate polyols and polyester polyols have been used in urethane oligomers. Polyester polyols are particularly attractive because of their commercial availability, oxidative stability and versatility to tailor the characteristics of the coating by tailoring the backbone. The use of polyester polyols as the backbone polymer in a urethane acrylate oligomer is described, for example, in U.S. Pat. Nos. 5,146,531, 6,023,547, 6,584,263, 6,707,977, 6,775,451 and 6,862,392, as well as European Patent 539 030 A.
Concern over the cost, use and handling of urethane precursors has lead to the use of urethane-free oligomers in coating compositions. For example, urethane-free polyester acrylate oligomers have been used in radiation-curable coating compositions for optical glass fibers. Japanese Patent 57-092552 (Nitto Electric) discloses all optical glass fiber coating material comprising a polyester di(meth)acrylate where the polyester backbone has an average molecular weight of 300 or more. German Patent Application 04 12 68 60 A1 (Bayer) discloses a matrix material for a three-fiber ribbon consisting of a polyester acrylate oligomer, 2-(N-butyl-carbamyl)ethylacrylate as reactive diluent and 2-hydroxy-2-methyl-1-phenyl-propan-1-one as photoinitiator. Japanese Patent Application No. 10-243227 (Publication No. 2000-072821) discloses a liquid curable resin composition comprising a polyester acrylate oligomer which consists of a polyether diol end-capped with two diacids or anhydrides and terminated with hydroxy ethyl acrylate. U.S. Pat. No. 6,714,712 B2 discloses a radiation curable coating composition comprising a polyester and/or alkyd (meth)acrylate oligomer comprising a polyacid residue or an anhydride thereof, optionally a reactive diluent, and optionally a photoinitiator. Also, Mark D. Soucek and Aaron H. Johnson disclose the use of hexahydrophthalic acid for hydrolytic resistance in “New Intramolecular Effect Observed for Polyesters: An Anomeric Effect,” JCT Research, Vol. 1, No. 2, p. 111 (April 2004).
While a number of optical fiber coatings are currently available, it is desirable to provide novel optical fiber coatings which have improved manufacturing and/or performance properties relative to existing coatings.